1. Technical Field
This invention relates to a power conversion apparatus used in a reactive power compensator connected to an electric power system.
2. Related Art
In a self-commutated reactive power compensator, a DC voltage increases when electric power flows due to a fluctuation in an AC system. Particularly when a system fault occurs, a voltage of a fault phase instantaneously increases at the time of the fault clearance and an excessive active power flows. In a conventional power conversion apparatus, feedback control of active current and reactive current or a DC voltage of a converter is generally performed, so that an inflow of an active power from an AC system is suppressed and an increase in a DC capacitor voltage is suppressed, but when an AC system voltage instantaneously increases, for example, in case of clearing a system fault, in conventional control, response was slow resulting in an increase in a DC voltage. When a response of a converter control is set to a high speed, current control operates effectively at the time of an inflow of an active power to suppress the inflow of the active power and the increase in the DC capacitor voltage can be suppressed. However, when the response is set to a high speed thus, there was a problem that divergence or vibration generally occurred at the time of the current control due to conditions of the system or a configuration of the self-commutated reactive power compensator and operations become unstable, and there was a limit to which the response would be set to a high speed. As a result of that, the power flows into due to a delay of the control and the DC capacitor voltage of the self-commutated reactive power compensator increases. When the DC capacitor voltage reaches a protective level of a DC overvoltage because of protection of a switching device of the converter, a DC overvoltage protective circuit of the converter operates and the converter leads to a protective stop. When the converter leads to the protective stop due to the DC overvoltage, the converter stops until the DC capacitor discharges. At this time, the converter stops for a long time unless the DC capacitor can be rapidly discharged. FIG. 9 shows a DC capacitor of a self-commutated reactive power compensator and the charger disclosed in the Unexamined Japanese Patent Application No. Hei 9-9509. In FIG. 9, numerals 1, 4 are self-commutated converters, and numeral 2 is a diode converter, and numeral 3 is an AC power source. In the self-commutated reactive power compensator shown in FIG. 9, conventionally, charge has made for initial charge of a DC capacitor of the self-commutated converter 1 using the diode converter 2 and instead, by replacing the diode converter 2 with the self-commutated converter 4 and operating an inverse conversion of the self-commutated converter at the time of an increase in a DC voltage, a DC overvoltage has been suppressed to prevent the converter from leading to a protective stop.
3. Problems to be Solved
As described above, in the conventional power conversion apparatus, there was a case that a response of converter control was set to a high speed and a large inflow of active power due to a fluctuation in a system could not be suppressed, and in such a case, there was conventionally a method of connecting the self-commutated converter as shown in the Unexamined Japanese Patent Application No. Hei 9-9509 in order to suppress an increase in a DC voltage and prevent a protective stop of the converter. Also, in addition to such a method, there was a method of connecting a switch and a resistor in parallel with a DC capacitor and performing a chopper operation at the time of an increase in a DC voltage to discharge the DC capacitor, or a method of reducing a variation in a voltage to an inflow of electric power by increasing capacitance of a DC capacitor. However, in these methods, a device which is not used in normal operation is added or a part with extra specifications is used for a product in the normal operation, so that a problem that costs increase arises.
The invention is implemented to solve the problems as described above, and an object of the invention is to provide a power conversion apparatus in which a stable operation is normally performed without adding a device which is not used in normal operation or using a part with extra specifications in the normal operation and only at the time of an inflow of excessive electric power from a system due to a fluctuation in the system, control can be constructed so as to operate at a speed higher than that of normal control and a converter can be prevented from leading to a protective stop due to a DC overvoltage.
(Means for Solving the Problems)
A power conversion apparatus of the invention is a power conversion apparatus connected to an AC system, comprising a current control circuit capable of individually performing feedback control of an active current and a reactive current, characterized in that the current control circuit is constructed so as to change a control gain of the active current according to a detected active current value or a deviation of a detected active current value from an active current command value.
Also, a power conversion apparatus of the invention is characterized in that in the current control circuit, the control gain of the active current is increased to a value larger than a normal value only when an excessive active current with a predetermined value or more flows in a rise direction of a DC voltage of a converter.
Also, a power conversion apparatus of the invention is a power conversion apparatus connected to an AC system, comprising a voltage control circuit capable of performing feedback control of a DC voltage of a converter, characterized in that the voltage control circuit is constructed so as to change a control gain of the DC voltage according to a detected DC voltage value or a deviation of a detected DC voltage value from a DC voltage command value.
Also, a power conversion apparatus of the invention is characterized in that in the voltage control circuit, the control gain of the DC voltage is increased to a value larger than a normal value only when the detected DC voltage value is a predetermined value or more.
Also, a power conversion apparatus of the invention is a power conversion apparatus connected to an AC system, comprising a current control circuit capable of individually performing feedback control of an active current and a reactive current, characterized in that according to a detected active current value or a deviation of a detected active current value from an active current command value, it is constructed so as to add a predetermined modulation factor bias to a normal modulation factor and output a voltage pulse signal to a converter.
Also, a power conversion apparatus of the invention is a power conversion apparatus connected to an AC system, comprising a voltage control circuit capable of performing feedback control of a DC voltage of a converter, characterized in that according to a detected DC voltage value or a deviation of a detected DC voltage value from a DC voltage command value, it is constructed so as to add a predetermined modulation factor bias to a normal modulation factor and output a voltage pulse signal to the converter.
Also, a power conversion apparatus of the invention is a power conversion apparatus connected to an AC system, comprising a voltage control circuit capable of performing feedback control of a DC voltage of a converter and means for detecting an AC system fault, characterized in that during the continuation of the AC system fault, it is constructed so as to decrease a DC voltage command value in the voltage control circuit.
Also, a power conversion apparatus of the invention is a power conversion apparatus connected to an AC system, comprising a voltage control circuit capable of performing feedback control of a DC voltage of a converter and means for detecting an AC system fault in the AC system, characterized in that during the continuation of the AC system fault and during a period of the time set after a clearance of the AC system fault, it is constructed so as to set an integral term of the voltage control circuit to zero when an absolute value of a deviation of a detected DC voltage value from a DC voltage command value becomes smaller than a setting value.